c-Jun N-terminal kinase (hereinafter abbreviated as JNK) is a protein kinase that belongs to the MAP kinase (hereinafter abbreviated as MAPK) family. Three JNK genes (JNK1, JNK2 and JNK3) have been discovered in mammals. Among these, JNK3 is selectively expressed, for example, in the brain-nervous system.
Unlike the classical MAPK, JNK3 is barely activated by proliferation stimuli. The activation of JNK3 is caused by stress on a cell (e.g., DNA damage, ultraviolet radiation, heat, high osmotic pressure, endoplasmic reticulum stress, active oxygen) or by an inflammatory cytokine (e.g., tumor necrosis factor (TNF), interleukin-1 (IL-1)). The activated JNK is understood to translocate from the cytoplasm into the nucleus and control the expression of target genes via the phosphorylation of transcription factors, such as c-Jun.
The activation of JNK is involved in apoptosis induced by various stress stimuli. For example, it has been reported that JNK is activated in nerve cell death resulting from withdrawal of nerve growth factor (NGF, non-patent document 1), and that nerve cell death resulting from withdrawal of NGF is suppressed by expression of a dominant negative c-Jun mutant (non-patent document 2). In addition, it has been reported that excitatory neuron death induced by administration of kainic acid is suppressed in a JNK3 knock-out mouse (non-patent document 3). The above findings suggest that the activation of JNK3 is involved in nerve cell death.
MKK4 and MKK7, which are members of the MAPK kinase family (hereinafter abbreviated as MAPKK), are known to activate JNK. MKK7 is also called MAPKK7, MAP2K7 and JNKK2, and specifically phosphorylates and activates JNK (non-patent document 4 and 5). In contrast, MKK4 phosphorylates and activates JNK as well as phosphorylates and activates ERK2 and p38, which are also members of MAPK family. Since JNK activation by osmotic pressure stimulus or ultraviolet radiation is still observed in an embryonic stem cell (ES cell) with a disrupted MKK4 gene (non-patent document 6), MKK7 is considered to work on the activation of JNK independently of MKK4.
The activation of JNK is also caused by a signal from cdc42, which is one of the low-molecular weight GTP proteins (non-patent document 7). PAK (p21 -activated kinase) is known to be a kinase that binds to cdc42 and transmits a signal from cdc42. Actually, it has been reported that the JNK signaling pathway is activated by over-expression of PAK1, PAK2, PAK3 or PAK4, all of which are members of the PAK family (non-patent document 8, 9, 7 and 10, respectively). However, the detailed mechanism of the signaling pathway between PAK and the JNK activation has not yet become clear. For example, whether the signaling is direct or indirect is not clear.
There have been some reports suggesting the involvement of cdc42 in nerve cell death. For example, forced expression of activated cdc42 in nerve cells induces nerve cell death, whereas a dominant negative cdc42 mutant suppresses nerve cell death resulting from withdrawal of NGF (non-patent document 11). In addition, it is reported that activated cdc42 activates MKK7 as well as activates JNK (non-patent document 12). Therefore, the signaling pathway from cdc42 to JNK, which is mediated by MKK7, may be possibly involved in nerve cell death.
Endoplasmic reticulum stress (hereinafter abbreviated as ER stress) is among the stresses causing the activation of JNK3. ER stress is caused by the accumulation of abnormal proteins in the endoplasmic reticulum (hereinafter may be abbreviated as ER) as a result of a defect in the protein folding process in the ER due to various stimuli (e.g., glucose exhaustion, change in homeostasis of calcium concentration, active enzyme). When ER stress occurs, expression of endoplasmic reticulum molecular chaperon is induced (that is, unfolded protein response: UPR), thus eliminating the misfolding. IRE1 is known to work in this process as an ER stress sensor protein (non-patent document 13).
It has been reported that IRE1 and TRAF2 are involved in the process of JNK activation caused by ER stress (non-patent document 14 and 15). Concretely, the IRE1 disrupted cell line shows suppressed JNK activation in response to ER stress, whereas over-expression of IRE1 activates JNK. In addition, IRE1 binds to TRAF2, and a dominant negative TRAF2 mutant suppresses the JNK activation by IRE1.
In addition to the above, JIK (also referred to as DPK) is also known as a protein that is involved in the process of JNK activation by ER stress. It is considered that JIK binds to IRE1 and TRAF2, and is involved in the JNK activation by ER stress. For example, over-expression of JIK augments the JNK activation by ER stress, whereas an active-site-deletion mutant of JIK suppresses the JNK activation by ER stress (non-patent document 15).
JIK is one of the STE20-related serine/threonine kinases that are human homologs of yeast Ste20p protein. In addition to the aforementioned actions of JIK, it has been reported that JIK inhibits the JNK activation by epidermal growth factor (EGF) stimulus, while the activity of JIK itself is suppressed (non-patent document 16), and that the over-expression of JIK leads to the JNK activation (non-patent document 17).
On the other hand, overload of ER stress is known to induce apoptosis. Since ischemia or accumulation of an abnormal protein such as polyglutamine or amyloid β (hereinafter abbreviated as Aβ) possibly gives rise to ER stress, it is pointed out that there is a relationship between nerve cell death due to ER stress and a neurodegenerative disorder.
Since apoptosis induced by ER stress was suppressed by each dominant negative mutant of MKK4 and MKK7 (non-patent document 18), JNK activation via MKK4 or MKK7 is likely to be involved in the apoptosis induced by ER stress.
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